Chris <chris.rowland@...> wrote:

I used generic TO220 style linear regulators for my linear SMPS
replacement, had to trim the tabs a bit but they are rated for 1A and
while they get warm seem OK.

Do you have a photo?

Stan Dye via groups.io <standye@...> wrote:

These two observations tell me that your problem is 'to the right' of the capacitor.

That's what I suspected as well.

After spending (too much) time on it, I think I figured out the source of
this buzzing (not the cause, just the source).

First, these 25 MHz oscillations were probably caused by the oscilloscope
probe. They're irrelevant.

There are 100 Hz spikes on inputs of two of four op amps (and, what's
interesting, on outputs of OTHER two op amps).

These spikes are not in sync with mains (in Poland I have 50 Hz mains, so
that was my first suspicion).

What in the QMX generates such spikes? Turns out that CLK0 and CLK1
signals are interrupted for 200 microseconds at this rate (100 Hz).
Why, I have no idea.

I replaced op-amps, replaced C416...C419 (I used 22n, I didn't have 33n,
but it didn't seem to make any difference), finally replaced C406 that you
mentioned (with 2x 1u, the same reason as above). Nothing changed.

So the source is located, and it would even make sense for these spikes to
appear on two of four outputs of IC403 (if the clock signal was always
interrupted in one or two "phases" of the detector -- but I didn't measure
both signals at the same time, so I'm only guessing).

I guess the proper question would be: could you / someone else look at
CLK0 and CLK1 lines (IC403 pins 2 and 14) and see if you see these breaks
there? It's hard to spot on the oscilloscope...

Maybe Hans can confirm that the CPU indeed does such thing? Or deny? Maybe
it's some weird firmware bug?

Have you checked carefully R401/R402/C406? A bad C406 in that biasing
circuit could cause oscillations and disturb the audio.

Yes, now I did (replaced C406). I didn't touch R401 or R402, as the
biasing voltage seems correct.

Are you certain that there is no anomaly with T401?

At this point I'm not sure of anything... but I don't think, given this
new discovery, it's T401. Plus these spikes are not present on its outputs
(pins 7 and 9 of IC403).

Is the buzzing only on 'left' or 'right' audio, or both?

Spikes are on inputs of IC406 (so right) and outputs of IC405 (so left).
Crazy.

And finally, as unlikely as it seems, IC407 has been the most common
failure source of anything related to audio in the QMX.? If none of the
previous apply, I would replace that one.

When I removed op-amps, the buzzing stopped, so I guess IC407 is fine...

btw - you are the third person now recently who I have heard blew the
IC403 with a strong overvoltage burning through the 5V supply, myself
being one of them (I inadvertently turned QMX+ on with a 17.5V supply).?
I don't know why the nearly identical IC402 didn't also blow.? Maybe
it's the doubled bias across the Z/W pins.

It would make sense. Maybe the chip can sustain overvoltage on VCC, but
not on Z and W (their input protection diodes would normally clamp it to
VCC, but with VCC also being too high it didn't happen)...

BTW, I recorded videos (with oscillograms) when I worked on it, partly in
case I needed some more advice, partly to document what I was doing in
case I got confused later. I summarized everything above, but here's the
raw footage anyway (not edited or cut, just merged).

Jerry Gaffke via groups.io <jgaffke@...> wrote:

Signals ADC_5V and ADC_3V3 back to the host should be a bit high by maybe a tenth of a volt.
If the processor doesn't see a high enough voltage,? it waits until it does before running any further code.
It's ok if those signals are a bit high, but there is no margin on the low side.

Thanks! It works now, but if my QMX ever stops booting, this will be what
I'll check first.

I'm amazed the 78M33 works at all, that's a lot of power to dissipate.

It's very hot, but didn't bang yet. I'm testing it with the case open now.
I'll see what happens when I close it.

I'd recommend either going with a bigger regulator (and a heat sink), or
feeding the 78M33 from your 5v linear..

78M33 and 78M05 are on the same heatsink (ground plane), so I guess it
won't matter which one dissipates the heat, as long as the heatsink can
absorb it...

I'd love to use a bigger regulator, but there's almost no space there. Or
is there? Did you fit a bigger regulator?

Gabe KO6EQN via groups.io <ggsubs@...> wrote:

Turn off break in (Vox button secondary function).
Adjust squelch to the maximum.

Which radio are you referring to?

Chris G7LQX via groups.io <mornymorny@...> wrote:

Is it possible to disable the RF input when using practice mode?? There
is significant background noise when practicing morse, since I can't
turn off the reception..... I guess a dummy load will help...?

I set the sidetone to absolute, volume to one step above muted, and if
there's still noise, lower the gain with my tool:



I just start it and press arrow down to decrease the gain to minimum, it
sends appropriate CAT commands.

Donald Adams via groups.io <djadams42@...> wrote:

Is it OK to apply power to a QMX when the LCD and Controls Boards are
removed?

Yes.

If so,? is it then OK to turn on the VDD supply by manually grounding
the PWR_ON line at the input connector of PCB #1?

Yes.

NOTE: This QMX is a 9V, user built, 60-15m unit that comes up with no VDD at power turn on.? If VDD did come up,

I assume the QMX?would be in the initial power up mode for loading firmware so only the linear IC101 VDD would be expected.

Is +12V (after power switch) present? Well, in your case it will be +9V,
but the line is still labeled +12V...

Is it OK to apply power to a QMX when the LCD and Controls Boards are removed?

?

If so,? is it then OK to turn on the VDD supply by manually grounding the PWR_ON line at the input connector of PCB #1?

?

NOTE:? This QMX is a 9V, user built, 60-15m unit that comes up with no VDD at power turn on.? If VDD did come up,

I assume the QMX?would be in the initial power up mode for loading firmware so only the linear IC101 VDD would be expected.

?

Don WB0LQW

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